The invention relates to an energy analyzer for charged particles in which the particles traverse an electric or magnetic field and to a detector system which is connected behind the analyzer and to which are fed the particles after they leave the field of the analyzer through an exit slit. The energy analyzer may be a spherical capacitor, a cylindrical capacitor, a magnetic sector field spectrometer or the like.
When experiments are conducted with prior art devices and samples of different sizes, there exists the danger that -- with small samples -- the substrate of the sample delivers interfering contributions to the spectrum of the sample. The resolution of a spherical capacitor, for example, is determined by the ratio of the width of the entrance and exit slits, or so-called apertures, to the radius of the spherical capacitor and the aperture of the beam in radial direction. However, the surface of the entrance aperture (width x length) and the aperture of the beam (total solid angle) are determinative of the luminosity (a measure of the sensitivity). The length of the apertures as well as the beam aperture in tangential direction are not included in a first aproximation of the resolution. This applies exactly for the ideal 180.degree. spherical capacitor. In order to obtain as high a sensitivity as possible it is desirable to have the entrance and exit apertures as long as possible. However, this length is limited by
1. the active surface of the detector system employed to detect the particles leaving the exit aperture; and PA1 2. limitations in the illumination of the entrance aperture of the spherical capacitor.
If, for example, in photoelectron spectroscopy the sample is reproduced in the entrance aperture of the spherical capacitor through a system of lenses, the maximum appropriate length of the entrance aperture is given by the length of the sample multiplied by the magnification factor of the lens. In order to attain optimum sensitivity, the aperture length should be adapted to the diameter of the available detector, generally a secondary electron multiplier.
The magnification factor of the lens is then adapted to correspond to the maximum significant sample size. If smaller samples are to be examined which are placed on a substrate, this substrate will inevitably deliver interfering contributions to the spectrum of the sample.